Why does the Mobile Safari Browser on iOS not allow file uploads?

As already known, it's not possible for iOS users to select image files to upload from Safari on iPhone, because the browse button to display the "select file"- dialog is disabled. It works fine on Android, but not on iPhone...

What is the particular reason for that issue? I heard that the browse button is disabled because there isn't a file browser on the iPhone. Someone other said that Safari just disabled root access. Is there any reliable source which explains the issue? (I need it for my thesis.)

iOS apps can only read data in their own directory and thus only Safari would be able to use these files - so how could you create them and put them in Safari's directories?

From Apple's Filesystem basics development documentation

An iOS app’s interactions with the file system are limited mostly to the directories inside the app’s sandbox. During installation of a new app, the installer code creates a home directory for the app, places the app in that directory, and creates several other key directories. These directories constitute the app’s primary view of the file system. Figure 1-1 shows a representation of the sandbox for an app.

Figure 1-1 Each iOS app operates within its own sandbox

Because it is in a sandbox, an app is generally prohibited from accessing or creating files in directories outside of its home directory. One exception to this rule occurs when an app uses public system interfaces to access things such as the user’s contacts or music. In those cases, the system frameworks handle any file-related operations needed to read from or modify the appropriate data store

iOS 6 fixed this problem by granting one exception to the sand boxing of apps from each-other's data. Upload buttons may now ask about taking/browsing for a picture or video clip to upload.

Other than by exception, there still is no full filesystem access on iOS 6 and lower.